Introduction

Silver iodide ( AgI) is one of the most extensively studied superionic conductors, systems characterized by having exceptionally high (liquid-like) values of ionic conductivity s in the solid state. This high conductivity is attained because of the diffusive motion of one of the constituent species through the sublattice formed by the particles of other species.
At room temperature and pressure AgI consists of a mixture of the g and b phases, which have the cubic zinc-blende and the hexagonal wurtzite structure, respectively. The b-phase becomes the more stable phase above 410 K, and a first-order structural phase transition to the superionic a-phase occurs at 420 K accompanied by an increase in s of about three orders of magnitude.

In the a-phase, the iodides form a body-centered cubic (bcc) lattice, while the silver ions diffuse through it. It has been demonstrated experimentally, by means of neutron and x-ray diffraction, and extended x-ray absorption fine structure studies, that the Ag+ ions occupy predominantly the tetrahedral sites located on the faces of the bcc unit cells, with the diffusion between these sites occurring mainly along [110] directions via trigonal sites.

The Influence of Polarization

The ionic induced polarization was not considered in the previous simulation works. However, Madden and Wilson showed the significant role of polarization in some ionic systems, like alkaline earth halides, and stated that it can account for phenomena often attributed to a certain degree of covalency. They proposed two different mechanisms for the induction of ionic polarization: a purely electrostatic induction governed by a fixed polarizability, and a mechanical short-range polarization that depends on the nearby ions. This latter contribution, that opposes the former, allows for the fact that the dipole moments created by the field are damped at short interionic separations where overlap effects become significant.

Models and Simulations

The purpose of this work is twofold. Firstly, we propose a new model that avoids the polarization catastrophe when both species are polarizable. In this model the short-range damping contribution opposes the dipole induced by the electric field created by both the ionic charges and the induced dipoles. The second aim is to complete the paper series about the polarization effects on the structure and ionic transport properties of molten silver halides. We have already studied molten AgCl and AgBr in previous papers, and we have recently published a preliminary MD study on the structure of molten AgI and the origin of the prepeak observed in the experimental structure factor.

In conclusion, computer simulation has been a powerful tool in the study of AgI properties, both in solid and molten phases. Our study provides further understanding of the behavior and properties of AgI, particularly the influence of polarization on its structure and ionic transport properties.